(1) Field of the Invention
the present method relates generally to mercury capture from gases, for example mercury capture from flue gases formed during combustion, such as coal combustion, and, in particular, to a new and useful system and method for capturing mercury using at least partially halogenated polymers such as halogenated polyamides.
(2) Description of the Prior Art
The burning of fossil fuels is estimated to release thousands of tons of mercury into the environment every year. Because of the toxicity of mercury, efforts are being made to reduce its release.
On Mar. 16, 2005, the United States Environmental Protection Agency (EPA) drafted its “Clean Air Mercury Rule” which required mercury reduction for existing and new coal-fired electrical power plants. This legislation on average required more stringent mercury reduction for bituminous coal fired power plants in the 28 most eastern states of the United States. The 7 states most impacted by the draft legislation follow:
United States Statewide Mercury Mass Balance 1999EPAReductionEPAReduction2010EPA 20102018EPA 2018Statelbs Hgplbs Hgxlbs Hgolbs HgTTons HgTTons HgT%Tons HgT%Pennsylvania590.405974.003394.209958.604.9791.7864%0.70286%Maine0.252.781.054.070.0020.00151%0.00151%Alabama170.922316.202440.204931.322.4661.28948%0.50979%Illinois70.342142.603776.205989.142.9951.59447%0.62979%Maryland97.501111.20611.401820.100.9100.4946%0.19379%West Virginia266.203004.601661.004931.802.4661.39443%0.55078%Ohio314.203620.203174.607109.003.5552.05742%0.81277%
Other states including New Jersey, Wisconsin, Massachusetts, Connecticut, Illinois, Michigan and New Hampshire have passed their own more stringent emission regulations for coal-fired power plants. It is known that flue gas desulphurization systems (FGDs) have the ability to capture about 63% and 87% of incoming total mercury (HgT) and oxidized mercury (HgX), respectively. Also, NOx controls systems such as selective catalytic reduction systems (SCRs) have shown some promise in reducing mercury emissions by catalytically oxidizing a portion of elemental mercury (Hgo) to oxidized mercury (HgX) that is more easily adsorbable by either certain types of fly ash or FGDs. However FGDs and SCRs are expensive and uneconomical for all but the largest power plants (e.g., 500 MWe or larger). The average size of American coal-fired power plants is about 275 MWe. Therefore there is a need to find non-FGD and/or non-SCR techniques for removing mercury from flue gas, especially in smaller coal-fired electric power plants (e.g., below 500 MWe capacity).
It is well known that halogenated activated carbon is suitable for use in adsorbing mercury from a flue gas including coal combustion flue gas. For instance, U.S. Pat. No. 6,953,494 to Nelson describes the use of halogenated carbonaceous sorbents, for example brominated powdered activated carbon for use in mercury removal from flue gas. One disadvantage of this technology is that some of the halogen-to-carbon bonds are weak, allowing halogen desorption from the carbonaceous material. For instance, activated carbons impregnated with iodine and iodine compounds are released from carbonaceous sorbents at moderately elevated temperatures. Thus, their use is largely limited to ambient temperature process streams (page 2, lines 30 to 33). Iodine compounds are primarily or only physically adsorbed on carbonaceous materials. (page 2, lines 37 to 39). Carbonaceous materials will both physically adsorb bromine species (Br2 and HBr) and chemically react with them. Physically adsorbed bromine is prone to desorb from the materials upon changed conditions such as injection into a hotter gas stream for example (page 8, lines 22 to 28). Brominating to 15 wt. % Br2 generally produces even more capable mercury sorbent, but as some of the bromine is held at less energetic sites, there is a greater possibility that some degree of bromine may evolve off under some circumstances (page 2, lines 50 to 54).
Further, the burning of fossil fuels is not the only source of mercury being introduced into the environment. For example, mercury, e.g., metallic mercury, is used extensively in a variety of industries, including many electrical, manufacturing and chemical industries.
Regardless of the source of the mercury, because of limited containment and recycling techniques, much of it ends up as environmental pollutant. Any amount of mercury in the environment causes concern because it may harm organisms and may accumulate in the human food supply. For example, methylmercury, which is lipophilic and highly toxic, is a common result of mercury being released into the environment. Because of its lipophilicity, it accumulates rapidly in the food chain. Further, because of its methyl group, methylmercury may be gaseous, and thus easily inhaled.
Thus, there is a need for reducing the release of mercury into the environment, as well as, to protect people from mercury that has already been released. More particularly, there is a need for techniques and products to help fossil fuel power plants comply with EPA standards, as well as, to prevent harm to organisms, including humans, and the environment by a new and improved system and method of mercury control.